Hypercholesterolemia is a causative agent of diseases including arteriosclerosis, atherosclerosis, cardiovascular disease and xanthomatosis. In addition, high serum cholesterol levels are seen in patients suffering from diseases including diabetes mellitus, various liver disorders, such as hepatitis and obstructive jaundice, familial hypercholesterolemia, acute intermittent prothyria, anorexia nervosa, nephrotic syndrome and primary cirrhosis. Improvement of lipoprotein profiles has been shown to retard the progression of such diseases, as well as to induce regression of clinically significant lesions in hypercholesterolemic patients.
To date, several classes of therapeutic agents have been used in the treatment of hypercholesterolemia. The first class of therapeutics, directed at reducing cholesterol absorption from the blood, include cholestyramine, colestipol, colchicine, neomycin, kanamycin, chloramphenicol, chlortetracycline and .beta.-sitosterol. The utility of such compounds is often limited by serious and frequent side effects, including aberrations in digestive function and malabsorption of essential nutrients. Furthermore, these drugs often induce a decrease in cholesterol absorption and an increase in cholesterol biosynthesis which, in turn, may effectively prevent or lessen the degree of total serum cholesterol reduction.
Agents which block the biosynthesis of cholesterol constitute another class of hypocholesterolemic drugs. Such compounds include mevinolin, synvinolin, eptastin, CPIB, atromid, methyl clofenapate, Su-13,437, halogenate, benzmalacene, DH-581 and MER-29. The administration of these synthesis blockers often leads to a toxic build-up of cholesterol precursors in blood and tissues.
Other classes of drugs used to treat hypercholesterolemia include those which act upon plasma lipoproteins (i.e., nicotinic acid), bile acid kinetics (i.e., choleic acid), .beta.-adrenergic receptors (i.e., propranolol), endocrine systems (i.e., growth hormones) and those with yet undetermined modes of action (i.e., pyridoxine and inositol).
The reduction of cholesterol level achieved by any of these classes of drugs is variable and rarely exceeds 25%, while the incidence of side effects may significantly limit their use.
As an alternative to pharmaceutical treatment of hypercholesterolemia, surgical procedures, such as partial ileal bypass and manipulation of the digestive tract, have been employed (J. Sabine, Cholesterol, pp. 237-40, Marcel Dekker Inc., New York (1977)). Attendant with such treatments, however, are the risks associated with invasive procedures. Although exercise regimens and dietary intake restrictions have been shown to reduce cholesterol levels, patient compliance with these types of therapy is often sporadic or insufficient to effect clinical improvement.
There is a low incidence of cardiovascular disease in populations consuming large amounts of cereal grains. Soluble and insoluble fibers have, in the past, been viewed as the agents responsible for cholesterol reduction in such populations (see D. Kritchevsky et al., "Fiber, Hypercholesterolemia and Atherosclerosis", Lipids, 13, pp. 366-69 (1978)). More recently, however, the hypocholesterolemic effects of cereal grains have been attributed to natural components of the grains--tocotrienols ("T.sub.3 ") and structurally similar compounds, such as tocopherols ("T"). For example, in U.S. Pat. No. 4,603,142, d-.alpha.-tocotrienol, isolated from barley extracts, was identified as an inhibitor of cholesterol biosynthesis. See also A. Qureshi et al., "The Structure of an Inhibitor of Cholesterol Biosynthesis Isolated From Barley", J. Biol. Chem., 261, pp. 10544-50 (1986)). Tocotrienols and tocopherols occur naturally in small quantities in plant sources, such as rice bran, palm oil and barley.
Tocotrienols are of special interest as cholesterol lowering agents, because they decrease the blood level of the low density lipoprotein fraction of cholesterol (LDL-cholesterol) and total serum cholesterol level, while increasing the ratio of the high density lipoprotein fraction of cholesterol (HDL-cholesterol) to LDL-cholesterol. Such effects are clinically significant, because HDL-cholesterol beneficially lowers the risk of heart disease (T. Gordon et al., "High Density Lipoprotein as a Protective Factor Against Coronary Heart Disease", The American Journal of Medicine, 62, pp. 707-14 (1977)).
Efforts to extract tocotrienols and tocotrienol-like compounds, such as tocopherols, from various grains, cereals and oils have resulted in the recovery of relatively small amounts of the desired compounds. For example, Canadian patent 480,484 refers to a method for preparing small amounts of tocopherol concentrates from a by-product of animal and plant oil processing called scum. Yields of tocopherols and tocotrienols according to United Kingdom patent application 2,090,836 are also low. It is believed that enzymes present in biological sources typically destroy tocopherols, tocotrienols and tocotrienol-like compounds during milling, extraction and other conventional processing techniques (see A. Qureshi et al., supra).
Accordingly, the need exists for processes which stabilize biological sources, thereby providing biological sources characterized by an increased content of tocotrienols, tocopherols and tocotrienol-like compounds and facilitating the isolation of tocotrienols, tocopherols and tocotrienol-like compounds from those sources in high yields.